1. Field of the Invention:
The present invention relates to a method for treatment with chemical solution and an apparatus for treatment with chemical solution, both used for etching or cleaning of semiconductor substrate in the process for production of semiconductor device or the like.
2. Description of the Related Art
When a semiconductor substrate is cleaned in the cleaning bath of a cleaning apparatus by a representative method using a mixed chemical solution of ammonia and hydrogen peroxide, the semiconductor substrate is cleaned at 60 to 80.degree. C., and the concentration of each component of the chemical solution in the cleaning bath decreases owing to the vaporization, decomposition, etc. of the component and changes with the lapse of time.
In a conventional cleaning method using a cleaning apparatus of FIG. 9, aqueous ammonia, hydrogen peroxide and pure water are fed, in necessary amounts, into a cleaning bath 101 from a storage tank 105 for aqueous ammonia, a storage tank for hydrogen peroxide and a storage tank for pure water, by the driving of solution-feeding pumps 108, 109 and 110; and a semiconductor substrate (not shown in FIG. 9) is cleaned with the resulting mixed chemical solution in the cleaning bath 101. The aqueous ammonia and the hydrogen peroxide are automatically supplemented into the cleaning bath 101 by given amounts at given time intervals.
In the above cleaning method, however, it is difficult to control the concentration of each component of the mixed chemical solution in the cleaning bath 101, at a constant level and, unless the automatic supplementation is optimized, the concentration of each component of mixed chemical solution changes largely with time and a large variation in said concentration takes place.
In Japanese Patent Application Kokai (Laid-Open) No. 278529/1992 is disclosed a cleaning apparatus as shown in FIG. 10, for keeping constant the concentration of each component of cleaning solution in cleaning bath. In the apparatus, a cleaning bath 151 is provided with an ammonia component analyzer 161 for measuring the concentration of ammonia and a hydrogen peroxide component analyzer 162 for measuring the concentration of hydrogen peroxide, and the concentrations of ammonia and hydrogen peroxide in the cleaning bath 151 are independently monitored by the analyzers 161 and 162 via a filter 154, a solution-feeding pump 153 and valves 163 and 164. A valve 155a connected to the storage tank 155 for ammonia and a valve 156a connected to the storage tank 156 for hydrogen peroxide are operated based on the signals sent from the analyzer 161 and the analyzer 162, respectively; thereby, the chemical component(s) whose concentration(s) in cleaning bath has (have) decreased, is (are) supplemented automatically and the concentrations of chemical components in cleaning bath are kept constant. Since the decrease in concentration of hydrogen peroxide is generally small as compared with the decrease in concentration of ammonia (caused by vaporization or the like), there is disclosed, in Japanese Patent Application Kokai (Laid-Open) No. 259141/1993, a cleaning method wherein only aqueous ammonia is supplemented depending upon the temperature used in cleaning and no hydrogen peroxide solution is supplemented. Further in Japanese Patent Application Kokai (Laid-Open) No. 142435/1995 are disclosed a cleaning method wherein cleaning is conducted in a concentration range at which the variation of etching rate relative to the variation of ammonia concentration is small, and a combination method thereof with the above-mentioned method of controlling the concentration of cleaning solution by the measurement of the concentration of each component of cleaning solution or with the above-mentioned method of supplementing only ammonia. Furthermore in Japanese Patent Application Kokai (Laid-Open) No. 22972/1996 is disclosed a cleaning method wherein the concentrations of hydrogen peroxide and ammonia are controlled at desired levels based on, for example, two parameters of cleaning solution, i.e. conductivity and pH.
The above techniques, however, had the following common problems.
When a mixed chemical solution of ammonia and hydrogen peroxide is used, there are present, in its fresh solution, various chemical species such as NH.sub.3, NH.sub.4.sup.+, H.sub.2 O.sub.2, HO.sub.2.sup.-, H.sup.+, OH.sup.-, H.sub.2 O and the like. In the solution which is being used for cleaning of Si semiconductor substrate, there are further present contaminants on semiconductor substrate and chemical species resulting from reactions between said contaminants and the components of chemical solution. Therefore, the cleaning ability of chemical solution during cleaning is determined by, for example, the concentrations of said chemical species, the reaction rates and free energies of the reactions of said chemical species, the temperature level of cleaning and the uniformity of concentrations of said chemical species in cleaning bath.
In any of the above-mentioned conventional techniques, only the concentrations of components in chemical solution were controlled and no control was made on the above-mentioned parameters for determining the cleaning ability of chemical solution, particularly the concentrations of dissolved chemical species and the reactions of dissolved chemical species. Thus, no control was made on the cleaning ability per se of chemical solution and it was impossible to efficiently conduct high-level control over cleaning of semiconductor substrate. Further, higher standards are being required for cleaning of semiconductor and the consciousness for environmental protection is increasing; in this connection, the demand for use of smaller amount of chemical solution has increased in recent years. The above-mentioned conventional techniques are not fully satisfactory for these requirement and demand.
For example, in the technique of Japanese Patent Application Kokai (Laid-Open) No. 278529/1992, when a chemical component large in concentration variation due to vaporization and/or decomposition is supplemented for maintenance of its concentration, the supplementation causes the dilution of other chemical component; as a result, the other chemical component must be supplemented as well. Consequently, the total amount of the chemical components supplemented into cleaning bath becomes large. That is, in order to keep the concentrations of chemical components at respective constant levels, it is necessary not only to conduct strict concentration control but also to supplement the chemical components in a large total amount. Meanwhile, the methods disclosed in Japanese Patent Application Kokai (Laid-Open) No. 259141/1993 and Japanese Patent Application Rokai (Laid-Open) No. 142435/1995 are applicable only under certain cleaning conditions and, depending upon the kind of material to be cleaned, make difficult the cleaning under said cleaning conditions.
The method disclosed in Japanese Patent Application Kokai (Laid-Open) No. 22972/1996 had similar problems as well because it controls the concentrations of hydrogen peroxide and ammonium hydroxide in chemical solution at desired levels. For example, when a component in chemical solution vaporized in a relatively large amount, there were cases that the component had to be supplemented in an amount unnecessarily larger than actually needed. The composition of chemical solution capable of giving a certain etching rate (surface-treating ability or chemical solution's cleaning ability) is not restricted to one particular composition and includes various compositions. Therefore, when the composition of chemical solution has shown a change, it is more efficient to conduct supplementation of chemical component(s) so that the composition after supplementation is close to the current composition and shows an etching rate (surface-treating ability or chemical solution s cleaning ability) closest to the initial etching rate, rather than to conduct the supplementation so as to regain the initial composition of chemical solution. This supplementation mode can reduce the cost for supplementation; moreover, the mode can shorten the time needed for supplementation and therefore the problem associated with the variation in composition of chemical solution during supplementation can be avoided. Thus, in the methods wherein the component concentrations in chemical solution are kept constant, there are cases that chemical components must be supplemented in amounts larger than actually needed; therefore, the methods had room for improvement in that respect.
In p. 719, 29p-E-14 of No. 2 of Extended Abstracts (The 44th Spring Meeting, 1997); The Japan Society of Applied Physics and Related Societies is presented a method (by the presenters including the present inventor) for predicting the etching rate of wafer surface when subjected to cleaning with ammonia-hydrogen peroxide solution, from the analysis of chemical equilibrium and subsequent analysis of etching reaction and also from the cleaning conditions used.
When such a method for prediction of etching rate is applied to control of chemical solution, the surface-treating ability of chemical solution, i.e. etching rate per se is controlled. Therefore, the method, as compared with conventional methods wherein the component concentrations in chemical solution are kept constant, makes possible more strict and more efficient control of chemical solution. In actual application of the method for prediction of etching rate, to control of chemical solution, however, it is the most important task how to efficiently determine, based on the method for prediction of etching rate, the optimum supplementation amounts of chemical components. That is, a means is required which can rapidly and accurately determine the optimum supplementation amounts of the components contained in chemical solution and effective for surface treatment. It is also a problem to solve how to determine the range of supplementable amounts of chemical components in relation to the constitution of the apparatus used and how to determine the actual amount of each chemical component supplemented, within said range of supplementable amounts. Further, in the method for prediction of etching rate, presented in the above Extended Abstracts, etching rate is expressed by a formula including, as the variables, the OH.sup.- concentration in chemical solution and the temperature of treating; therefore, an experimental formula must be prepared each time when the conditions of surface treating, the conditions of substrate surface, etc. vary; thus, the method had room for improvement with respect to the accuracy of prediction of etching rate and the applicability of the method.